Since the first introduction of the power chain saw various types of safety devices have been added to the tool to help insure against injury to the tool operator. Tool kickback is an inherent hazard in the operation of this type of power tool and kickback may occur when the tip end of the cutting chain, which is mounted about a chain guide bar, comes in contact with an object not cutable by the chain and/or by a pinching action of the guide bar during a cutting operation. Both of these operational conditions cause reaction forces which may result in the operator losing control of the tool and this posses a hazard and a very real danger to the operator.
Power chain saws have conventionally included a pair of handle controls each of which is gripped by the operator to control the tool. A rearward handle includes various switches and/or button controls which function to operate the saw motor. A top handle is mounted and positioned such that the operator may grip it to maintain directional control over the tip end of the saw. When the cutting chain is stopped momentarily by an uncutable object in its path and/or the tip end of the guide bar is pinched, the reaction force of the cutting chain causes a rotation of the saw in a direction opposite to the chain movement about the guide bar. This reaction force may effect a movement of the chain guide bar up and back in an uncontrolled arc which is mainly in the plane of the guide bar. Under some operating conditions, the guide bar may move in an arc toward the operator who must have decisive control of the tool by way of the rear and top handles.
In light of the chain saw kickback hazard, various type of chain brake devices have been suggested and/or added to the tool such as to automatically stop cutting chain motion about the guide bar when kickback occurs. Chain brake mechanisms of the type alluded to have taken the form of a band brake which operates in conjunction with the motor drive hub which includes a clutch and sprocket of the chain drive. The band brake is tightened about the peripheral extent of the hub by a forward movement of the saw top handle or, by a forward movement of a safety guard which may be mounted in a relative position to the top handle when such are provided art as exemplified in U.S. Pat. Nos. 4,077,125; 4,335,514; 4,426,563; 4,586,588; and 5,882,844.
In addition to a chain braking device, it has been suggested that motor shut-down at the time of chain brake engagement would also be a beneficial safety advantage in the operation of these type power tools. In this respect, a mechanical linkage is disclosed in U.S. Pat. No. 4,426,563 which operates in conjunction with the saw top handle and the motor on/off switch such that, when saw kickback occurs, a rotation of the top handle effects motor shut-down by a reactive movement of the linkage. U.S. Pat. No. 4,335,514 also discloses a connecting link element which communicates with the saw top handle and the motor on/off switch such that any occurrence of saw kickback will automatically move the switch to the motor "off" position. This is obviously necessary when the chain saw is powered by an electric motor. In addition to these prior art teachings, U.S. Pat. No. 4,882,844 discloses a chain brake device which operates in conjunction with an electrical solenoid which automatically enables the chain brake when the saw trigger control is released.
Mechanical linkages of the type taught by the prior art obviously require a redesign and/or a re-arrangement of the conventional parts which may be found in presently available power chain saws Furthermore, any of the earlier designed chain saws which are being used in the field will require an alteration and/or modification to accommodate these additional mechanical arrangements. However, such modifications to existing saws may jeopardize any warranty rights in the tool when not specifically authorized by the saw manufacturer.
Thus, there is an obvious need in the art for a power chain saw all-stop safety device which is fool-proof, easily adapted to presently designed saws and, which requires very little or no modification to those chain saws already being used in the field. Further, there is a need in the art for a power chain saw safety device which is low in cost and which effectively produces motor shut-down almost instantaneously when chain brake engagement is made.